InfraRed Atmospheric Sounding Interferometer (IASI)

Introduction

IASI is the main payload instrument of the MetOp platform dedicated to support Numerical Weather prediction (NWP) and meteorological purposes. Besides delivering temperature and humidity data, IASI also acquires information on more than 25 other atmospheric components with high precision and is supporting the efforts of scientists monitoring Earth atmosphere composition.

The IASI instrument observes and measures twice a day the spectrum of thermal infrared radiation emitted by the Earth from a low altitude sun-synchronous polar orbit. The use of IASI data in NWP accounts for 40% of the impact of all space based observations in NWP forecasts. There are currently two IASI instruments in operation: on MetOp-A (launched 19 October 2006) and on Met-Op B (launched 17 September 2012) with the third due for launch in 2018. IASI will be followed by IASI-New Generation (NG), on-board MetOp-SG, with demonstrated performances by a factor of 2.

Under an agreement between the French space agency “Centre National d’Etudes Spatiales” (CNES) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), the former was responsible for developing the instrument and data processing software. The latter is responsible for archiving and distributing the data to the users, as well as for operating IASI itself. A technical expertise centre at CNES’s Toulouse Space Centre is tasked with monitoring the IASI system’s performance and handling any anomalies. Data are processed on the ground by Eumetsat. France is playing a pre-eminent role in the broad scientific community exploiting data from the IASI mission.

Mission objectives

The first objective of IASI is to meet the needs of the European meteorological centres and EUMETSAT. But a second major objective is also to foster the use of the data for climate (green-house gases and clouds mainly) and air quality monitoring by supporting research experts and activities in these areas.

The IASI measurements were then designed to be compatible in terms of sampling, resolution, accuracy and overall performances with the mission objectives of providing information on:

Temperature and water vapour information for Numerical Weather Prediction (NWP) models and climate research

Atmospheric composition through critical species including O3 – Ozone and other key trace gases

Heritage

Data from polar orbiting satellites have been made freely available to the worldwide meteorological community by the USA for more than thirty years and on a fully operational basis for nearly two decades. Before 2000, the National Oceanic and Atmospheric Administration (NOAA) was responsible for the related satellite programmes by supporting two meteorological missions, one in a morning, the other in an afternoon orbit. NOAA indicated then that it would not be able to maintain the two satellite system – which is considered as the minimum for meteorological applications – beyond the year 2000.

Following discussions between the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and NOAA about future cooperation regarding operational meteorological satellites, it has been agreed that EUMETSAT will take over the provision and operation of the Metop series of satellites in a morning orbit while NOAA would continue to provide and operate satellites in an afternoon orbit.

The operational NOAA High Resolution Infrared Radiation Sounder (HIRS) offered a spectral resolution of about 10 cm-1 and an accuracy of around 2.5 Kelvin for temperature and 40% for humidity.

The IASI project was identified as a priority by CNES during the Scientific Prospective Seminar in cap d’Agde (France) in 1989. EUMETSAT issued the requirements for an operational interferometer thermal infrared sounder in 1991. The related phase B started in 1996, and phases C/D in 1998. The IASI Science Plan has been prepared by members of the IASI Sounding Science Working Group, (ISSWG), a group established by CNES and EUMETSAT in 1995 with the objective of providing the scientific preparation for the IASI mission, under the coordination of its chairmen.

Instrument description

Built by Thales Alenia Space with technical oversight from CNES, IASI comprises two main elements: a spectrometer designed to resolve atmospheric infrared radiation into line spectra, and an imager to locate points to be sounded. The sophisticated nadir-viewing spectrometer is a Fourier Transform Spectrometer (FTS) based on a Michelson Interferometer coupled to an integrated imaging system. The optical interferometry process offers fine spectral samplings of the atmosphere in the infrared band between wavelengths of 3.4 and 15.5 um (from 645 to 2760 cm−1) at 0.25 cm−1 resolution (0.5 cm−1 after apodisation). It has 8461 spectral samples that are aligned in 3 bands within the spectral range.

With a swath width on the Earth’s surface of about 2000 kilometres, global coverage is achieved in 12 hours. For optimum operation, the IASI measurement cycle is synchronised with the American AMSU (Advanced Microwave Sounding Unit) instruments. The elementary (or effective) field of view (EFOV) is defined as the useful field of view at each scan position. Each such element consists of a 2×2 circular pixel matrix of what is called instantaneous fields of view (IFOV). Each of the four pixels projected on the ground is circular and has a diameter of 12 km at nadir. The shape of the IFOV at the edge of the scan line is no longer circular: across track, it measures 39 km and along track, 20 km

Atmospheric composition products

The IASI products cover a broad range of applications. The temperature in the troposphere and lower stratosphere is retrieved under cloud-free conditions with a vertical resolution of 1 km in the lower troposphere; a horizontal resolution of 25 km, and an accuracy of 1 kelvin. The humidity of the troposphere is derived under cloud-free conditions, with a vertical resolution of 1–2 km in the lower troposphere; a horizontal resolution of 25 km, with an accuracy of 10%.